The present invention relates to a sheet post-processing unit such as a sorter for distributing and accommodating printed sheets discharged from the image forming machine, and to an image forming apparatus having the sheet post processing unit and an image forming machine such as a copying machine, a printer and the like for forming images on a printing sheet.
Various types of sheet post-processing units for distributing printed sheets which have images formed thereon and discharged from a stencil printing machine have heretofore been developed and put to practical use. Sheet post-processing units of the sorts mentioned above are desired to be devised so that while every possible effort is made for size reduction, a larger number of printed sheet of, for instance papers may be distributable.
In such a sheet post-processing unit of a fixed bin type, a plurality of bins are fixedly arranged in the vertical direction of a casing and conveyer units fitted with fans and blowers are installed in the vertical direction of the plurality of bins and besides indexers as sheet guide means for carrying printed sheets into the corresponding bins are moved up and down vertically along travel routes of the conveyer units. Therefore, an attempt has been made to reduce the size of the whole apparatus by decreasing the dimension thereof in the depth direction. With respect to the conveyer unit, moreover, the diameter of a corner portion at both ends of a belt has also been designed for its size to be minimized. However, the following problems still exist because various types of sheet are used as printing sheet in the stencil printing machine cooperating with aforementioned sheet post-processing unit.
When it is attempted to use the belt for conveying the printed sheet discharged from the stencil printing by means of the sheet post-processing unit thus arranged while the printed sheet is being drawn to the belt, the travel route is largely curved in the corner portion of a belt 94a in a belt conveyer unit 94 as shown in FIG. 11.
More specifically, no problem will particularly be caused even though the uppermost bin 78A is first used for accommodating the printed sheet in a case where the printed sheet discharged from the stencil printing machine and conveyed thereto is ordinary paper. Nevertheless, the firm thick printed paper tends to easily peel off the belt 94a after it has made a U-turn in the corner portion of the belt 94a and passed thereon and it also becomes readily afloat and deviates from the travel route even when a suction unit 96 is employed for drawing the printed paper to the belt. Consequently, the printed paper is conveyed at an acute angle with respect to an indexer 116 as shown by an arrow A of FIG. 11 and caused to strike against the surface of the indexer 116 in a manner sticking therein. Therefore, the printed paper becomes unaccommodated in a target bin 78, which also poses a problem causing a paper jam.